Problem: The scale on a map is 4cm : 3km. If two cities are 20cm apart on the map, what is the actual distance in km between the two cities?
Answer: The scale means that every 4cm on the map corresponds to an actual distance of 3km. A distance of 20cm on the map is the same as 5 $\cdot$ 4cm. The actual distance between the two cities is 5 $\cdot$ 3km, or 15km.